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The Singleton Pattern is used to make sure only one object of a particular class is ever created. This can be useful when when exactly one object is needed to coordinate actions across a system; perhaps for efficiency where creating lots of identical objects would be wasteful, perhaps because a particular algorithm needing a single point of control is required or perhaps when an object is used to interact with a non-shareable resource.

Weaknesses of the Singleton pattern include:

It can reduce reuse. For instance, there are issues if you want to use inheritance with Singletons. If SingletonB extends SingletonA, should there be exactly (at most) one instance of each or should the creation of an object from one of the classes prohibit creation from the other. Also, if you decide both classes can have an instance, how do you override the getInstance() method which is static?

It is also hard to test singletons in general because of the static methods but Groovy can support that if required.

Example: The Classic Java Singleton

Suppose we wish to create a class for collecting votes. Because getting the right number of votes may be very important, we decide to use the singleton pattern. There will only ever be one VoteCollector object, so it makes it easier for us to reason about that objects creation and use.

it has a private constructor, so no VoteCollector objects can be created in our system (except for the INSTANCE we create)

the INSTANCE is also private, so it can't be changed once set

we haven't made the updating of votes thread-safe at this point (it doesn't add to this example)

the vote collector instance is not lazyily created (if we never reference the class, the instance won't be created; however, as soon as we reference the class, the instance will be created even if not needed initially)

To support lazy-loading and multi-threading, we could just use the synchronized keyword with the getInstance() method. This has a performance hit but will work.

We can consider variations involving double-checked locking and the volatile keyword (for Java 5 and above), but see the limitations of this approach here.

Example: Singleton via MetaProgramming

Groovy's meta-programming capabilities allow concepts like the singleton pattern to be enacted in a far more fundamental way. This example illustrates a simple way to use Groovy's meta-programming capabilities to achieve the singleton pattern but not necessarily the most efficient way.

Suppose we want to keep track of the total number of calculations that a calculator performs. One way to do that is to use a singleton for the calculator class and keep a variable in the class with the count.

First we define some base classes. A Calculator class which performs calculations and records how many such calculations it performs and a Client class which acts as a facade to the calculator.

Now we can define and register a MetaClass which intercepts all attempts to create a Calculator object and always provides a pre-created instance instead. We also register this MetaClass with the Groovy system:

Guice Example

We can also implement the Singleton Pattern using Guice. This example relies on annotations. Annotations are a Groovy 1.1 feature and will need to be run on a Java 5 or above JVM.

Consider the Calculator example again.

Guice is a Java-oriented framework that supports Interface-Oriented design. Hence we create a Calculator interface first. We can then create our CalculatorImpl implementation and a Client object which our script will interact with. The Client class isn't strictly needed for this example but allows us to show that non-singleton instances are the default. Here is the code:

Note the @Inject annotation in the Client class. We can always tell right in the source code which fields will be injected.

In this example we chose to use an explicit binding. All of our dependencies (ok, only one in this example at the moment) are configured in the binding. The Guide injector knows about the binding and injects the dependencies as required when we create objects. For the singleton pattern to hold, you must always use Guice to create your instances. Nothing shown so far would stop you creating another instance of the calculator manually using new CalculatorImpl() which would of course violate the desired singleton behaviour.

In other scenarios (though probably not in large systems), we could choose to express dependencies using annotations, such as the following example shows: